1. Field of the Invention
The present invention relates to a fingerprint image input apparatus for inputting a fingerprint image of a finger surface to a computer or the like.
2. Description of the Related Art
In recent years, along with the development of an information-oriented society, security techniques for personal verification have received a great deal of attention to aim at control of areas as intelligence and security for important areas and access control to computer terminals. A method using an ID card or a password is very popular as a method of verifying a person's identity for a specific person. This system has limited safety, and therefore demand has arisen for developing a more safe system which is easy to operate.
In general, fingerprints have been used to verify person's identity due to its major features, i.e., "unchangeability throughout life" and "no identical patterns", and high verification precision can be obtained. In a conventional method, fingerprint photographs are used to identify fingers, and the photographic patterns are discriminated by human beings. In recent years, however, fingerprints tend to be discriminated and identified by computers due to the development of electronic techniques. For this purpose, image input apparatuses for accurately inputting fingerprint information to computers are required, and many proposals have been made and are being made.
Although various methods have been proposed in conventional fingerprint input apparatus, most popular apparatuses optically detect signals of fingerprint information and processing the fingerprint signals as two-dimensional signals. Another conventional method, such as forming of multi-value projection signals in the longitudinal direction of the finger, is also proposed ("Method of Verifying Person's Identity", Takeda, Uchida, Hiramatsu, and Matsunami, Technical Research Report in Japan the Institute of Electronic and Communication Engineers: PRU 89-50). According to this method, when only one-dimensional signals are used, the data volume can be greatly reduced as compared with a two-dimensional signals of fingerprint image, and a processing algorithm can be simplified. For this reason, a signal processing speed can be increased, and the time required for verifying person's identity can be shortened. This method is rarely affected by ridges of a skin surface of a finger, i.e., finger ridge lines, broken lines, and adhesion. The "fingerprint" in this specification represents the whole or part of ridges on finger skin, which include patterns on the skin surface of the finger as a whole.
Image input apparatuses for optically reading fingerprints are based on the following three methods due to their principles of operation.
The first method is "total reflection method" (Japanese Patent Application No. 42-9347: Fingerprint Verification Apparatus).
The second method is "light-path separation method" (Japanese Patent Application No. 57-26153: Three-Dimensional Pattern Information Detecting Method).
The third method is "scanning method" (Japanese Patent Application No. 53-130600, Fingerprint Processing Apparatus; and Japanese Patent Application No. 56-183189, Method and Apparatus for Processing Fingerprint).
These conventional methods pose the following problems. More specifically, in the first method (total reflection method) or the second method (light-path separation method), an object lens and an image input device are required to accurately receive the fingerprint image. The lens and the image input device (i.e., those elements which constitute an image input device) consequently are generally expensive in cost. In order to form multi-value projection signals in the longitudinal direction of the finger, all data of the entire finger must be input. In order to input all the data of the entire finger in the form of image signals by one operation, a long distance between the object lens and the finger is required because the operation is determined by the image input device and an object lens used together with the image input device.
For example, assume that an image pickup element (having a light-receiving surface of 8.8 mm.times.6.6 mm) equivalent to a 2/3 inch element is used as an image input device, and that data of a 50-mm long finger are input using an object lens having a focal length of 16 mm. Under these assumptions, a minimum distance between the object lens and the finger is about 90 mm. When the total size of the object lens and the image input device, that is, the size of the image input apparatus, is also taken into consideration, the fingerprint image input apparatus as a whole is expected to become bulky. This limits easy installation and free transportation of the apparatus.
In the first method, light from a light source incident on an image input apparatus through the object lens is light totally reflected on the surface of a transparent member. This condition is equivalent to a case wherein an observer watches the light source, and the screen is very bright. A fingerprint image obtained by a finger pressed on the surface of the transparent body is obtained by light scattering upon tight contact of the projection surface of the fingerprint with the transparent body. For this reason, when an amount of light reflected by a portion of the fingerprint is decreased, a dark image viewed in the bright field of view has a low contrast level, and minute portions of the fingerprint cannot be easily read.
The surface on which the finger is pressed must be an optically very flat polished surface in order to improve the characteristics of total reflection. Fat and moisture are attached to the surface of the transparent member every time the apparatus is used. When fat and moisture which have refractive indices similar to that of the transparent member are attached to the total reflection surface upon incidence of light, light is not totally reflected at a boundary between the clean surface of the transparent member and a surface portion with a fat and is transmitted through the fat. Since the boundary between the fat and the air is not flat due to the surface tension of the fat, light is reflected or scattered at the boundary between the fat and the air. The fat and moisture on the surface of the finger, which are left and attached to the surface of the transparent member every use emphasizes a dark image of scattered light in the totally reflected bright field of view. This scattered image overlaps the original image obtained upon tight contact between the finger and the surface of the transparent member, thereby further decreasing the contrast level of the original image.
In the second method, a bright image of scattered light is formed in a dark field of view. In principle, since this image is formed by scattered light, an amount of light of the image portion is small and S/N ratio is reduced, but a decrease in contrast as in the first method does not occur in the second method because a portion surrounding the scattered light image is a dark field of view. However, since light incident on the image input apparatus is limited to light scattered by the ridges of the skin surface of a finger, the resultant image is susceptible to a change in ambient brightness. When fat is left on the transparent surface which totally reflects light from a light source, an image formed by scattered light appears as in the first method. Upon reflection, the scattered light does not reach the image input apparatus. The light is transmitted through the fat without being totally reflected at the boundary between the transparent member and the fat, and light reaches the boundary surface between the fat and the air. Since the boundary surface between the fat and the air is not a flat surface due to the surface tension of the fat, the total reflection condition cannot be satisfied, and light is reflected by an amount which is determined by a difference of refractive indices of the air and the fat. The reflected components are incident again on the transparent member from different directions. Some components reach the image input apparatus. An image formed by these components overlaps the true fingerprint image and becomes noise against the true fingerprint image.
Since the third method (scanning method) is a method of mechanically scanning a light beam, an optical system and a mechanical system are required to focus a beam into a beam spot. These systems are optically and mechanically complicated. Alternatively, a special light source such as a laser for focusing light into a small beam spot and an expensive lens almost free from aberration must be used. In addition, it takes a long period of time to receive signals.
Even if any one of the first to third methods is employed, a common problem is presented. That is, a multi-value projection image (i.e., a signal having ridge information representing different shapes) must be formed from the two-dimensional image signals of the entire finger. This processing requires a lot of information to form image signals of the entire finger. A complicated processing algorithm is required to process signals so as to obtain a multi-value projection signal. As a result, it takes much time to perform the above signal processing.
As described above, many problems described above are posed by the conventional methods. A multi-value projection signal in the longitudinal direction of the finger is formed from the image signals of the entire finger. The one-dimensional signals are extracted as finger characteristics, i.e., as signals for verifying person's identity. This method requires a large amount of data to obtain the image signals of the entire finger, and the complicated algorithm is required to perform signal processing to obtain a multi-value projection signal from the image signals. It takes a long period of time to perform signal processing.
In order to input the image signals of the entire finger by one operation, a long distance between the object lens and the finger is required due to the limitations of the image input devices and the object lens. As a result, the apparatus as a whole becomes bulky.
The conventional apparatuses for inputting finger images (i.e., apparatuses based on the first and second methods) has a low contrast level or a low S/N ratio due to the residual fat.
The third method requires a special light source such as a laser which can be focused to form a very small beam spot and an expensive lens almost free from aberration. As a result, the structure is complicated
at high cost.